All machines designed for packaging palletised loads, irrespective of the specific type, present the problem of sealing the plastic film used for wrapping the palletized load at the end of the wrapping operation. The appropriateness of this operation can be readily appreciated in so far as it has the purpose of preventing, during subsequent displacements of the load, the free end of the film from possibly getting caught up, so causing problems, such as, for example, instability of the palletized load or unwinding of the wrapping. Currently, the most widespread methods for sealing plastic film consist in an operation of heat sealing, which constrains the free end of the film to the last layer of the wrapping, or else in the insertion of the free end of the film used for wrapping the palletized load within a sort of pocket obtained by setting at a distance from the load the layers of the film that envelops it.
The first of these operations is frequently carried out using an apparatus for infrared irradiation; the temperature of the film is brought up to approximately 70° C., at which point there is softening of the film and sealing of the free end to the last layers of the wrapping. Alternatively, there is used a contrast bar, which is temporarily inserted underneath the last layers of the wrapping, as well as an appendage at high temperature that compresses the free end of the film against the contrast bar, welding it to the wrapping. However, since in general these machines operate in dusty and crowded environments, the use of high temperatures constitutes a constant source of danger.
Furthermore, with specific reference to the use of these packaging machines in the foodstuff sector, the following factors can prove particularly inconvenient: the production of fumes, which often occurs in the welding of plastic film; the vicinity of the foodstuffs, which is often destined to undergo freezing, to areas at particularly high temperatures; and infrared radiation, with which at least part of the foodstuffs packaged are in any case irradiated.
The second operation is obtained with a mechanical apparatus characterized by the simultaneous action of a spacer member, which separates the last layers of the film from the underlying ones, and an insertion member, which pushes the free end of the film into the pocket formed by the spacing of the film at a distance from the load.
The main limit of the above process is linked to the difficult synchronization of the two devices, operation of which, controlled by pneumatic pistons, must take into account the marked elastic characteristics of the film: in other words, it may occur that even a slight variation (in the region of the tenth of a second) of the times of operation of the pistons will cause an early release of the pocket or of the free end of the film. In the first case, which is the less serious one, the spacer member can remain imprisoned in the pocket, but, thanks to an appropriate shaping, can slide out just by leaving a slight decrease in the tensioning of the film; in the second case, instead, and with rather more serious consequences, the free end of the film would escape, rendering the sealing operation vain.